Admin-editable helper text to clarify passwordless sign-in screen

Allow community admins to add a short custom line of text to the sign-in screen. Heartbeat is used for the paid membership community, and members arrive from the website's "Member Login" link at community.newvolutions.com; signed-out visitors land on a screen that asks for their email. Because sign-in is passwordless, existing members often think it is a "create account" or sign-up page and hesitate or leave. Suggested example text: "Already a member? Enter the email you joined with and we'll send you a sign-in link." It would be even better if the sign-in screen had a clear "Sign in" heading and a separate "Join" link that points to the community's landing page.